F150 ENGINE SWAP QUESTION

1973 Ford F-150

Engine Mechanical problem
1973 Ford F150 Four Wheel Drive Automatic

Bought a 1973 Ford F100 Truck with a 390 and four wheel drive. With gas prices I want to switch out the 390 and go to an inline six. Do I have to replace bell housing, transmission, engine or is any of it compatible with the 6? Will the transfer case of the four wheel drive fit to an inline 6 or will I have to change that to. Found plenty of info on going bigger in swap, found nothing on going smaller in engine swap. Thanks.